A woman who suffered horrific burns after a medical mishap has been offered a $50 fuel voucher by way of apology.

The 21-year-old received severe internal burns when undiluted acid was used by mistake during a procedure at Wakefield Specialist Centre in Wellington.

Newton Mall Pharmacy, which labelled the acid, is investigating what went wrong.

The pharmacy sent the woman the fuel voucher with a letter of apology.

The traumatised woman said the letter was of little comfort.

She told the Herald on Sunday she endured "horrific pain" while the undiluted solution was applied to her. But she thought the pain was part of the procedure.

The report says the woman felt the voucher was a slap in the face.

The woman is now seeking compensation from Newtown Mall Pharmacy for her medical bills.

Her family has also lodged a complaint with the Health and Disabilities Commissioner (HDC).

The HDC confirmed on Friday a complaint had been laid but would not comment further.